1. Field of the Invention
The present invention generally relates to cardiac pacers, and more particularly to a dual chamber rate responsive pacemaker which uses activity sensing to differentiate true exercise induced sinus tachycardia from atrial arrhythmias and retrograde atrial events, and to permit a selective ventricular rate control.
2. Background Art
Early pacemakers were asynchronous (VOO), and they stimulated the heart at a fixed rate, independent of the patient's underlying cardiac rhythm or metabolic demand. Although such pacers, typified by U.S. Pat. No. 3,057,356 to Greatbatch, provide a ventricular pacing rate sufficient to sustain life, this pacing mode can compete with native ventricular rhythms which is undesirable.
Subsequently, demand pacemakers (VVI) were developed. This type of pacer interacts with the patient's heart to provide stimulating pulses only if spontaneous cardiac activity does not occur. An example of such a pacer is taught by U.S. Pat. No. 3,478,746 to Greatbatch. This form of pacer provides a ventricular sense amplifier for detecting ventricular depolarizations. A ventricular sense event resynchronizes the pacer's V-V timer by resetting and restarting it. The ventricular sense event also cancels or inhibits the scheduled ventricular stimulus and thus avoids competition with the native ventricular rhythm.
Atrial synchronized pacers (VAT) were developed almost simultaneously with VVI demand pacemakers. This type of pacer paces the ventricle in response to the detected atrial rate of the patient. The VAT pacer, as typified by U.S. Pat. No. 3,253,596 to Keller, provides an atrial sense amplifier for detecting atrial depolarizations. An atrial sense event starts the pacer's A-V delay timer. When the A-V timer times out, a ventricular stimulus is provided. Conceptually, such a pacer can be considered as a prosthetic conduction pathway which simulates the natural A-V conduction pathways of the heart. One drawback to this form of pacing is its ability to compete with ectopic ventricular activity. An ectopic ventricular beat (PVC) may be detected in the atrium which will result in the generation of a ventricular stimulus a short time after the ventricular depolarization. Although such a pacing regime is considered harmless if the A-V delay is short, it is possible to deliver the pacing stimulus into the vulnerable period of the ventricle if a premature ventricular contraction occurs, and thereby initiate an arrhythmia.
Continued development of pacemakers was marked by the invention of the AV sequential pacer (DVI), as disclosed in U.S. Pat. No. 3,595,242 issued to Berkovits. This form of pacer provides for stimulation in both the atria and the ventricles while providing sensing in the ventricle. In this DVI mode pacer, a ventricular sense event starts both a V-A escape interval, the pacer delivers an atrial stimulus, and at the end of the V-V escape interval, the pacer delivers a ventricular stimulus. If a ventricular sense event occurs during the V-A or V-V time intervals, the pacer will resynchronize to the ventricular sense event and inhibit the delivery of the otherwise scheduled ventricular stimulus.
The DDI mode pacer described by U.S. Pat. No. 3,747,604 to Berkovits further includes an atrial sense amplifier to inhibit the atrial stimulus if an atrial sense event occurs during the V-A interval. The atrial sense event does not resynchronize the pacer which makes this device especially suitable in patients where atrial competition must be avoided.
The atrial synchronized ventricular inhibited or VDD mode pacer, as disclosed in U.S. Pat. No. 3,648,707 issued to Greatbatch has structures for sensing in the atrium and ventricle but provides stimulating pulses only in the ventricle. In operation, the VDD pacer will synchronize on detected atrial activity and provide a ventricular stimulus if one does not occur within the A-V delay initiated by the atrial depolarization. Ventricular sense events inhibit the delivery of the otherwise scheduled stimulus and resynchronize the pacer's V-V timer.
The dual sense--dual pace DDD mode pacers, have been described in U.S. Pat. No. 4,312,355 to Funke. The DDD pacer addresses many of the shortcomings of the prior art devices. The DDD mode pacer, as described by Funke, has had wide applications. This type of pacer has sense amplifiers for detecting both atrial and ventricular events, as well as output pulse circuits for stimulating both the atrium and the ventricle.
This form of prior art pacer provides timing circuitry to initiate an A-V delay upon the occurrence of an atrial event. If, during the A-V delay period, no spontaneous ventricular event is sensed, the pacer will produce a ventricular stimulus at the conclusion of the A-V delay period. If, during the V-A interval, no spontaneous atrial event is sensed, the pacer will produce an atrial stimulus at the conclusion of the V-A interval.
In this type of pacemaker, in the absence of spontaneous P waves and R waves, the heart will be stimulated at fixed AA and VV intervals with a fixed AV delay. However, if the ventricle depolarizes spontaneously, then the A-V is truncated and the observed A-A interval is not fixed and will be shorter than the arithmetic sum of the programmed A-V and V-A intervals.
The dual chamber modalities, DVI, VAT, VDD and DDD, have proven to be especially efficacious pacemakers since they restore A-V synchrony and thus improve cardiac output by accommodating the hemodynamic contribution of the atrial chambers within the pacing regime. The latter three modes also synchronize the pacing rate to the patient's native atrial or sinus rate and thus provide an increased pacing rate in response to bodily activity. Increasing cardiac rate is the major contributor to increased cardiac output.
More recently, other pacers which increase cardiac output in response to exercise have been proposed. They include pacemakers which rely upon the sensing of a historical average of atrial activity, blood pH, respiratory rate or QT interval data to alter the pacemaker's escape interval. A discussion of these background proposals may be found in "The Exercise Responsive Cardiac Pacemaker", IEEE Transactions on Biomedical Engineering, Vol. 12, December 1984.
One approach which is important to the understanding of the present invention is the activity responsive pacer described in U.S. Pat. No. 4,428,378, issued to Anderson et al, and which is incorporated by reference. The pacer disclosed in that patent monitors the physical activity of the patient and increases the pacing rate in response to increasing patient activity.
Other publications that provide background information for the operation of the present invention include U.S. Pat. No. 4,890,617 issued to Markowitz et al. which is incorporated herein by reference. This patent describes a dual chamber activity responsive pacemaker which senses and paces in both the atrium and the ventricle. The pacing rate is determined by the sensed activity of the patient, the programmed lower rate, and the patient's atrial or sinus rate.
U.S. Pat. No. 4,932,046, entitled "Dual Chamber Rate Responsive Pacemaker", assigned to Medtronic, Inc. of Minneapolis, Minn., which is incorporated herein by reference, describes a dual chamber rate responsive pacemaker. The pacemaker operates in an atrial synchronized modality when the sensed atrial rate is within a physiological range, and paces at a sensor-determined rate when the atrial rate is above or below the physiological range.
The Goy et al. article "Rate Response Dual Chamber Pacing" in Centro Editoriale Italiano, 1986, pages 60-65 describe in general terms, the clinical results of a rate responsive dual chamber pacemaker.